After a constricted portion of, e.g., a coronary artery is expanded by a blood vessel expanding catheter, in order to prevent reconstriction of the expanded portion, an inner diameter retaining instrument (stent) for ensuring the inner diameter of the lumen of a tubular organ has been proposed.
A conventional inner diameter retaining instrument disclosed in Japanese Patent Publication (Kokoku) No. 61-6655 employs a unidirectional shape memory alloy. This alloy is formed into a tubular shape having an inner diameter substantially equal to an inner diameter of a normal blood vessel to memorize the shape. The alloy is then deformed to reduce its outer diameter so as to be easily inserted in the blood vessel. The resultant structure is then introduced to a desired position in the blood vessel. Thereafter, the structure is heated by warm water and is restored to the memorized shape.
The above conventional inner diameter retaining instrument consisting of a unidirectional shape memory alloy, however, cannot be deformed unless an external force is applied thereto once the instrument is restored to its memorized shape and is expanded. For this reason, even if an affected portion is restored after indwelling of the inner diameter retaining instrument, the instrument cannot be removed, thus posing a serious problem in terms of adaptability with a living body. In addition, even if it is found upon expansion of the inner diameter retaining instrument that the instrument is indwelled at a wrong position, it is very difficult to change the indwelling position.
It is an object of the present invention to provide an instrument for retaining the inner diameter of a tubular organ lumen, which can be caused to freely contract even after it is expanded in a tubular organ, thereby enabling recovery from an indwelling position and arbitrary change in indwelling position upon expansion.